1. Field of the Invention
The present invention relates to dot printers and more particularly to a printing head of a dot printer.
2. Description of the Prior Art
FIG. 1 shows a conventional example. In the figure, numeral 1 designates a support member provided with a plurality of electromagnets 2. Each electromagnet 2 comprises a coil 4 mounted on a yoke 3 and an armature 5 rotatably mounted thereon. On the support member 1 is mounted a cover 7 on which a stopper 6 is supported to determine the reset position of the armature 5.
A guide holder 8 is attached to the support member 1 using screws 9. In the front surface and the center of the guide holder 8 are fixed needle guides 11, 12 which align a plurality of needles 10 and slidably hold them, and in the rear portion thereof is fixed a plate-shaped spring support 14 which allows the needles to pass through and receives one end of needle springs 13 with coil spring. The other end of each needle spring 13 is contacted with a cap 15 fixed to the rear end of the needles 10. A platen 18 is installed on the front surface of the needle guide 11 and holds a paper 17 opposite a printing ribbon 16. A specific needle slides by means of attracting action of the armature 5, and thus printing is performed.
In assembly, however, before mounting the armature 5 the needles 10 slide by force of the needle springs 13 so that the top end of the needles 10 go back from position C of the needle guide 11 and the rear end further projects backwards from attraction position A of the armature 5, as shown in FIG. 2. It is therefore impossible for all needles 10 to pass through the needle guide 11 and all armatures 5 are held at a pushing state to mount the stopper 6. Accordingly, the armatures 5 must be temporarily fixed one at a time so as to insert the needles 10 in the needle guide 11 and then released from the temporary fixing state after mounting the stopper 6, resulting in a quite troublesome assembly. If one stopper 6 determines the reset position of one armature 5 only, the temporary fixing work of the armature 5 may be omitted. In this conventional example, however, the number of components increases and installation of many stoppers 6 causes the number of assembling steps to increase. If a needle spring 19 having large spring constant is used as shown in FIG. 3, the top end of the needle 10 is pulled out of the needle guide 11 and the rear end moves beyond the attraction position A of the armature 5 by a relatively small amount, thereby facilitating assembly of the armature 5 and the stopper 6. However, as the needles 10 come close to the platen 18 during printing, the load of the needle spring 19 increases significantly. Unless the distance between the needle guide 11 and the platen 18 is not reduced, printing cannot be carried out, an the reduced distance causes the paper 17 to be soiled by the printing ribbon 16. This invention has disadvantages also in that the electromagnet 2 of large capacity is required and the power consumption increases.